This proposal consists of 4 Specific Aims that will investigate the perceptual consequences of selected cochlear and stimulus manipulations. The overall goal of the proposed research is to document the extent to which changes in hearing function, as measured psychophysically in behaving animals, can be related to changes as the physiological status induced or measured in the auditory system. The first Aim will investigate the perceptual consequences of electrical stimulation of the round window, as also studied in Projects 3 and 6. This project will determine the extent to which the acoustic component generated by such stimulation is perceived as a high-fidelity representation of the input waveform. The second Aim will examine the perceptual consequences of infusion of neuroactive compounds into the fluids of the inner ear; in particular, of substances that are agonists or antagonists for glutamate receptors which also are studied in Project 4. For example, in appropriate doses, AMPA, a glutamate receptor agonist will induce swelling of afferent nerve endings, and will produce a change in sensitivity which can be reversed when the drug infusion is ceased. In the proposed studies, osmotic pumps will be used to infuse the substances into the scala tympani of awake animals that have been trained as psychophysical observers, and measures of changes in auditory function will be observed as the drug regimen is varied. The third Aim will examine the effects of efferent inactivation in a subject trained to attend to differences in multiple dimensions between a variety of auditory stimuli. The psychophysical task and stimulus contrasts will be optimized for demonstrating differential efferent effects, and reversible efferent inactivation techniques will enable multiple manipulations in each subject. For the first 3 Aims, an important feature of the research is documentation of the relationship between changes in function and structural changes determined from morphological evaluation. The fourth Aim will examine changes in the detection and localization of sound sources as a function of various spatial configurations of signal source and noise masker. These studies complement physiological studies of spatial hearing being carried out in Project 1.